Radiation standard

ABSTRACT

Low pressure mercury vapor lamps utilizing a relatively high current of approximately 0.5-1.5 A/cm2 provide within carefully controlled operating parameters a highly reliable radiation standard for 1849 A.U. far ultraviolet light which is insensitive to many design parameters.

United States Patent [191 Johnson 1*Nov. 18, I975 RADIATION STANDARD [56] References Cited [75] Inventor: Peter D. Johnson, Schenectady, UNITED STATES PATENTS N.Y. 3,679,928 7/1972 Johnson...-. 313/225 [73] Assignee: General Electric Company, Schenectady, Przmary Examiner-John Komlnskl Attorney, Agent, or FirmJerome C. Squillaro; Joseph Notice: The portion of the term of this Cohen patent subsequent to July 25, 1989, has been disclaimed. [57] ABSTRACT [22] Flled: 1972 Low pressure mercury vapor lamps utilizing a rela- [21 App], N 227 941 tively high current of approximately 0.5-1.5 A/cm provide within carefully controlled operating parameters a highly reliable radiation standard for 1849 A.U. [52] US 315/94; 313/225; 313/226 far ultraviolet light which is insensitive to many design 51 Int. c1. HOlJ 61/12 parameters. [58] Field of Search 313/225, 226; 315/94 6 Claims, 2 Drawing Figures .8. Patent Nov. 18, 1975 RADIATION STANDARD This invention is directed to new and unique apparatus for the provision of a far ultraviolet wavelength standard. More particularly, the invention is directed to apparatus suitable for providing an accurate and insensitive standard for ultraviolet radiation at a wavelength of 1849 AU.

The lighting arts and timing arts, among others requiring precisely defined standards, are in great need of intensity standards, particularly in the far ultraviolet. Thus, for example, there is a need for such a standard in calibrating the response of spectroradiometers and measuring the absolute intensity of the sources of radi ation in the vacuum ultraviolet. The most recently available calibrated wavelength standard, an incandescent quartz envelope tungsten iodide-regenerativecycle lamp, is suitable for wavelengths in excess of 2500 AU. There are, however, no available standards for wavelengths below that value.

Accordingly, it is an object of this invention to provide a wavelength standard for 1849 AU. far UV radiation.

It is another object of the invention to provide a wavelength standard in the far-UV which is insensitive to the usual variables to which wavelength standards are normally responsive.

Briefly stated, in accord with one embodiment of the present invention, I provide a far-UV radiation standard in the form of an elongated evacuable envelope containing a low pressure of an inert gas and a suitable quantity of mercury sufficient to provide, when ionized and operated under normal lamp operating conditions, a mercury are having a current density within the range of 0.5 to 3.0 ampereslcm Means are also provided to serve as are electrodes to maintain such a discharge and to adjust the current supplied, and the voltage applied, to such electrodes in order to regulate the arcing current so as to provide the required current density.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may better be understood with reference to the following detailed description taken in connection with the appended drawing in which:

FIG. 1 is a schematic vertical cross-sectional view of a wavelength standard lamp constructed in accord with the present invention and including means for supplying voltage to the appropriate electrodes for maintaining the proper operating parameters therefor, and

FIG. 2 is a similar schematic vertical cross-sectional view, with parts broken away, of an alternative embodiment of the discharge standard lamp of the present invention.

A unique feature of the present invention is that when the critical parameters of gas pressure, discharge current density, and temperature of operation are satisfied, other criteria of the design of the lamp which constitute the wavelength standard are quite variable. Thus, for example, the exact shape of the lamp and, within certain limits, the dimensions thereof do not critically affect the operation of the lamp. Similarly, various types of arc electrodes suitable for carrying the required current density may be utilized without departing from the spirit of the invention or affecting its wavelength or intensity output. Accordingly, it is to be understood that the illustrations described herein are representative only of certain embodiments of the device which may be constructed for purposes of description.

FIG. 1 illustrates a simplified far-ultraviolet radiation standard lamp in accord with the present invention. The lamp of FIG. 1 includes an evacuable envelope represented generally at 10 which includes an ultraviolet transmissive central member 11 and a pair of enlarged electrode-containing end members l2, 13 respectively. Each of end members l2, 13 includes a cathode assembly 14, which in this instance comprises an externally heated assembly which is mounted between the inboard ends of a pair of inleads l5 and 16 and has connected therebetween and parallel with the filament-like member thereof a pair of auxiliary elec trode members l7, 18. These members, on alternate cycles of an alternating current voltage exciting the device under alternating current operation, serve as anode and cathode members respectively to sustain an electric discharge. The desired current and voltage to operate the lamp are supplied by a power supply means capable of supplying the requirements of the lamp in operation and also capable of limiting the discharge current so that when the diameter of the elongated member 11 is taken into consideration, the current density within the lamp falls within the prescribed range to obtain the insensitive, highly reliable standard of radiation provided thereby.

One means for providing voltage to the arc electrodes of assemblies 14 located in the end members of the envelope 10 may comprise, as is shown, a saturable transformer represented by dotted line box 19. Transformer 19 includes a primary winding 20 and a voltage step-up secondary winding 21 having a pair of tapped secondary low voltage portions 22 and 23 which are connected across respective pairs of electrode inleads l5 and 16 at either end of the lamp so as to provide alternating currentheating of each of the filaments 14 by the voltage developed across the tapped secondary. This voltage is necessarily low, of the order of several volts, to cause external heating of the filament to sustain a sufficient degree of thermionic emission of electrons to support an electric discharge between the filament and anode members at respective ends of the arc tube. The filaments as described herein are functionally similar to conventional fluorescent lamp filaments although the design is not critical. As is mentioned hereinbefore, many types of electrodes may be used in the standard lamp of the present invention and it is to be understood the standard fluorescent lamp electrodes are similar and may be used.

The interior of envelope 10 is charged with a sufficient quantity of vaporizable mercury which is sufficient to provide, in operation, a proper vapor pressure of mercury to sustain the arc required to carry the desired and necessary current density to facilitate operation of the wavelength standard. Additionally, a quantity of an inert gas which may conveniently be from 1 to 20 Torr but which is preferably for ideal operation of the order of 2-5 Torr and which is a noble gas, at these pressures argon is preferred, however, for improved rejection of extraneous radiation. In operation, the lamp is started by the application of a line voltage which may conveniently be volts or 230 volts at 60 cycles to the primary 20 of transformer 19. This causes ionization of the inert gas to establish a discharge which, in turn, ionizes some of the vaporized mercury within the lamp. The lamp immediately, therefore, becomes operative within the required parameters of operation of the lamp. The mode of operation of the lamp constituting the intensity standard herein is similar to that described in my copending applications Ser. No. 50,203 now US. Pat. No. 3,657,591; Ser. No. 50,105 now US. Pat. No. 3,657,590; and Ser. No. 50,106, now US Pat. No. 3,679,928 all of which were filed June 26, 1970 and assigned to the instant assignee. Those lamps were directed to the generalized concept of far ultraviolet radiators emitting wavelengths below 2000 A.U. and operating at current densities of from approximately l ampere/cm to an excess of 100 amperes/cm and were for general purposes.

I have since discovered that when the operating parameters of such high current density, low pressure lamps are further carefully chosen and operated within a narrow range, such that the current density is maintained from approximately 0.5 to 3.0 ampere/cm' and the pressure of the mercury vapor within the lamp is within the range of 0.025 to 0.25 Torr, resulting in operating temperatures normally within the range of 45 to 90C. the lamps emit far-ultraviolet radiation at a wavelength of 1849 A.U. which is approximately ten times the intensity of the 1942 A.U. emission and of substantially constant intensity. Thus, the far U.V. emission of the lamp is nearly purely the 1849 A.U. mercury resonance line.

A careful study of this mercury resonance line emission has shown that under the predetermined and hereinbefore described operating parameters, the line undergoes substantial broadening so that self absorption by unexcited mercury atoms is minimized so that the intensity is nearly independent of unexcited mercury atom density near the walls of the lamp envelope, fur ther resulting in the stability of the standard. Developmental tests which I have conducted indicate that the intensity of the emission of the lamp under the predetermined operating conditions is such as to maintain very constant, i.e.. varying only from approximately 0.015 to 0.03 watts/cm over the entire current density of 0.5 to 3.0 amperes/cm and is constant irrespective of the diameter of the ultraviolet transmissive radiation emitting portion of the lamp from dimensions ranging from 5 to millimeters ID. and for discharge paths of approximately 5 to 100 centimeters between the arc electrodes. Preferably, however, the most stable range is from 1.0 to 1.5 A/cm My work has further shown that, despite the fact that emission of other mercury resonance lines, namely, the 1942 A.U. and the 2537 A.U. line, vary greatly in intensity with the temperature of operation from 20 to 120C the intensity of emission of the lamps operated in accord with the design parameters of the present invention varies only approximately 20% over this range and is essentially constant to within 5% from the range of to 90C. Preferably, however, for temperatures of about to C the variation in intensity is only approximately 2%.

Due to the foregoing demonstrated characteristics which I have determined, the radiation standard lamps of the present invention are particularly useful since, with the output in intensity as a function of current density being nearly saturated in the operating range, highly accurate control of lamp current is not necessary. Therefore, conventional line voltages may be used and conventional fluorescent lamp ballast transformers may be used. Slight fluctuations in temperature do not change the operating characteristics of the standard. Similarly, other variations may be tolerated, the use of the ballast transformer being cited by way of example, not by limitation. Since the radiant flux density of the radiation standard lamp is substantially independent of arc tube diameter for any given current density, the accurate control of discharge tube diameter is not necessary and lamps may be designed in unique conformations to accommodate particular uses, a characteristic not normally found in radiation standards.

Another important feature of the lamp set forth herein is the substantial independence of the flux of radiation intensity from operating temperature. Normally the operating temperature will seek a value within the less than 5% variation range of 4590C. However, should it be necessary under unique circumstances to provide thermal means to maintain a moderately constant temperature range of operation a great latitude of design variation is permissible.

One particular radiation standard lamp which is constructed in accord with the present invention utilized a tube, as illustrated in FIG. 1 and utilized a pair of conventional watt rapid start fluorescent lamp cathodes separated by a discharge space of 25 centimeters. The envelope was fabricated of Spectrosil fuzed silica having an inside diameter of 1 centimeter in the UV radiation-transmissive section. A GE 7G3908 rapid start fluorescent lamp ballast operating at 0.8 amperes was connected across a 60 cycle, volt line and yielded a current density within the lamp envelope of 1.01 amperes/cm? The 1849 A.U. radiant standard operated to provide an output of 1849 A.U. at a radiant flux of 0.022 watts/cm of discharge envelope surface area.

In FIG. 2 of the drawing an alternative construction for the device of FIG. 1 is illustrated. In FIG. 2 the evacuable envelope is represented generally at 29 and includes an ultraviolet transmissive central portion 30 having three serpentine curves and five substantially straight sections each of which is transmissive to ultraviolet radiation or which may be made nontransmissive, as may be desired.

It should be appreciated that, as is set forth hereinbefore, a wide variety of possible variations and design parameters permutations and combinations may be made freely. It is desirable, however, that, irrespective of the design between the arc electrodes, an idealized discharge path of from 10100 centimeters, and preferably of the order of 25 centimeters, be maintained.

End portions 31 and 32 of envelope 29 contain arc electrodes adapted to be operated with direct current excitation. The are electrodes include a cathode assembly 33 and an anode assembly 34. Cathode assembly 33 may conveniently have a so-called M-l cathode structure, that is, a dispenser type filament member 35 containing a single loop and fabricated from a mesh stocking container containing a particulate mass of a thermionic emissive substance, such as barium aluminate or lanthanum boride, for example. A cathode shield member 36 laterally surrounds cathode member 35 and contains an aperture 37 therein for the escape of electrons to sustain an electric discharge along the axis of the serpentine lamp envelope.

Anode assembly 34 contains a collector means which, as illustrated, is a hollow cup 39 supported upon inlead 40 which passes through a pinch seal 44 in the end 32 of envelope 29. Similarly, dispenser filament 35 in end member 33 is supported upon inleads 45 and 46 which pass through pinch seal 47, which also supports a support member 48 upon which the cathode assembly is stabilized.

Although direct current excitation may be utilized in accord with the present invention, it is not necessary to use the same in order to obtain very high current densities, since the current densities of the lamp are within the moderate range as compared with the lamps of my co-pending applications. It is within the contemplation of the invention, however, that the standard be adaptable for use with direct current. For a high precision standard, regulated d.c. operation is preferred.

Furthermore, it will be appreciated that although particular electrode structures have been shown both in FIGS. 1 and 2, it is within the contemplation of the invention that various types of electrodes other than that shown herein may be utilized.

While the invention has been disclosed herein with respect to particular embodiments thereof and certain structures have been descibed by way of example, it is readily apparent that many modifications and changes will readily occur to those skilled in the art. Accordingly, by the appended claims I intend to cover all such modifications and changes as fall within the true spirit and scope of this disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A reference standard of emission of 1849 A.U. UV radiation emitting a uniform intensity of 1849 A.U. radiation'under varying operating conditions and comprising a. an evacuable discharge envelope including as a rations to provide a steady state partial pressure of mercury within said envelope within the range of 0.025 to 0.25 Torr;

d. a pair of arc electrodes dispersed atelectrical ends of said discharge envelopes and adapted to sustain therebetween an electric discharge within said envelope; and

e. means electrically coupled with said electrodes for controlling said electrical discharge to maintain said discharge at a current density within the range of 0.5 to 3.0 amperes/cm and establish therein an operating temperature within the range of 45*90C.

2. The apparatus of claim 1 wherein said discharge current is maintained at a value of approximately 1 ampere/cm 3. The apparatus of claim 1 wherein said arc electrodes are separated by a distance of approximately 5 to centimeters.

4. The apparatus of claim 3 wherein said distance is approximately 25 centimeters.

5. The apparatus of claim 1 wherein said discharge has an intensity of approximately 0.015 to 0.03 watts/cm of discharge envelope wall.

6. The apparatus of claim 1 wherein said intensity is approximately 0.022 watts/0m 

1. A REFERENCE STANDARD OF EMISSION OF 1849 A. U. U. V. RADIATION EMITTING A UNIFORM INTENSITY OF 1849 A. U. RADIATION UNDER VARYING OPERATING CONDITIONS AND COMPRISING A. AN EVACUABLE DISCHARGE ENVELOPE INCLUDING AS A RADIATING PORTION THEREOF A UV TRANSMISSIVE SECTION OF RESTRICTED DIAMETER; B. A GASEOUS FILLING WITHIN SAID ENVELOPE OF A NOBLE GAS WITHIN THE RANGE OF 1-20 TORR; C. A QUANTITY OF MERCURY WITHIN SAID ENVELOPE SUFFICIENT UPON THE ATTAINMENT OF LAMP OPERATING CONDITIONS TO PROVIDE A STEADY STATE PARTIAL PRESSURE OF MERCURY WITHIN SAID ENVELOPE WITHIN THE RANGE OF 0.025 TO 0.25 TORR; D. A PAIR OF ARC ELCTRODES DISPERSED AT ELECTRICAL ENDS OF SAID DISCHARGE ENVELOPES AND ADAPTED TO SUSTAIN THEREBETWEEN AN ELECTRIC DISCHARGE WITHIN SAID ENVELOPE; AND E. MANS ELCTRICALLY COUPLED WITH SAID ELECTRODES FOR CONTROLLING SAID ELECTRICAL DISCHARGE TO MAINTAIN SAID DISCHARGE AT A CURRENT DENSITY WITHIN THE RANGE OF 0.5 TO 3.0
 2. The apparatus of claim 1 wherein said discharge current is maintained at a value of approximately 1 ampere/cm2.
 3. The apparatus of claim 1 wherein said arc electrodes are separated by a distance of approximately 5 to 100 centimeters.
 4. The apparatus of claim 3 wherein said distance is approximately 25 centimeters.
 5. The apparatus of claim 1 wherein said discharge has an intensity of approximately 0.015 to 0.03 watts/cm2 of discharge envelope wall.
 6. The apparatus of claim 1 wherein said intensity is approximately 0.022 watts/cm2. 